The invention relates to a process for the production of benzene and at least one isomer of xylenes that is selected from among metaxylene, paraxylene and orthoxylene, starting from a feedstock that comprises aromatic hydrocarbons with 8 carbon atoms per molecule. The process applies particularly to the synthesis of very pure paraxylene for producing an intermediate petrochemical product, terephthalic acid. The process also applies to the synthesis of benzene that is used in particular for producing phenylalkanes by alkylation of benzene with mono-olefins, whereby the phenylalkanes constitute compounds for the formulation, after sulfonation, of biodegradable detergents.
The production of the specific isomer of xylenes is a significant petrochemical process in the synthesis of polyesters, used in particular in the fabric manufacturing industry. It then is important to be able to produce the desired isomer, preferably paraxylene, with maximum purity. Several techniques for separating isomers have been developed. Thus, the separation of the isomers can be done by adsorption, for example in a zeolitic sieve, which delivers a fraction that is very high in paraxylene and a fraction that is low in paraxylene and therefore high in orthoxylene and metaxylene, in the presence of an elution solvent. Since the composition of the aromatic feedstocks with 8 carbon atoms varies broadly according to their origin, however, whereby the content of para and ortho isomer generally approaches 50% relative to the total amount of xylenes, a single process does not make it possible to maximize the production of the desired xylene isomer. It is then necessary to combine an adsorption stage of the feedstock followed by an isomerization stage of this fraction that is low in the desired isomer as is described in Patent GB 1 420 796. Patent EP 531 191 of the applicant describes a process for the production of paraxylene by treatment in an adsorption zone followed by a crystallization stage of the adsorbed paraxylene, whereby the raffinate that is low in paraxylene is sent into an isomerization zone.
During this isomerization stage, the ratio of isomers at equilibrium is reestablished since the desired isomers are produced at the expense of the remaining isomers. The catalyst that is used in the isomerization reactions can be mordenite mixed with other zeolites such as the ZSM-5 zeolite as described in U.S. Pat. Nos. 4,467,129; 4,482,773 and EP 138 617 B. Other catalysts are based on mordenite and have been described in, for example, U.S. Pat. Nos. 4,723,051, 4,665,258 and FR 2,477,903.
Isomerization catalysts of C8-aromatic fractions that are based on EUO-structural-type zeolites, i.e., the EU-1, TPZ-3 and ZSM-50 zeolites, have been described by the applicant in Patent Applications FR 97/16,456 and 97/16,458.
Furthermore, the xylenes can be obtained by the transalkylation reactions of fractions that comprise toluene and aromatic hydrocarbons with 9 carbon atoms, as described in Patent FR 2,761,905 of the applicant, in the presence of a catalyst with a mordenite and binder base that comprises at least one metal that is selected from among the metals of groups VI, VIII and VII. Actually, the transalkylation/dismutation makes it possible to move methyl groups of the aromatic compounds with 9 carbon atoms to compounds such as toluene to obtain xylenes. When the catalyst has dealkylating properties, ethylbenzene can also be transformed into benzene.
Some aromatic fractions with 8 carbon atoms contain a large amount of ethylbenzene, which results in the use of processes that link the production of benzene and the production of paraxylene. It is known by U.S. Pat. No. 4,783,568 to combine the stages of separation of the xylenes, isomerization and transalkylation to transform the ethylbenzene of this type of feedstock.
Based on the variety of products that are introduced into the isomerization zone, however, it is important to be able to optimize the isomerization conditions of the isomers of the xylenes, and thereby to reduce the impurities and increase the paraxylene yield.
This invention relates to a scheme of three processes that have as their object to produce an isomer of xylenes starting from a feedstock that comprises aromatic compounds with 8 carbon atoms (C8), including ethylbenzene. The feedstock is treated in a zone for separating isomers of xylenes to extract the desired isomer, the raffinate that is low in desired isomer is sent into a zone for catalytic isomerization of the xylenes, the mixture that is obtained, which has a xylene composition that is close to the equilibrium composition and to which toluene and aromatic compounds with 9 carbon atoms (C9) are added, is sent into a catalytic transalkylation zone to recover, after treatment in at least one distillation zone, a fraction that is high in benzene, a fraction that is high in toluene that can be recycled at the inlet to the transalkylation zone, a fraction that is high in xylenes that is sent into the zone for separation of xylenes and a fraction that is high in aromatic compounds with 9 carbon atoms that can be recycled at the inlet to the transalkylation zone. The scheme according to the invention is characterized in that the isomerization catalyst comprises at least one EUO-structural-type zeolite, for example the EU-1 zeolite and at least one element of group VIII of the periodic table. In an implementation of the invention, the fraction that is high in benzene can be sent to a catalytic zone for alkylation of benzene with mono-olefins.
The process according to this invention makes it possible, surprisingly enough, to obtain paraxylene yields that are much higher than those of the prior art using improved performance levels in activity and selectivity during the isomerization stage, which also leads to a reduction of the recycling volumes as well as a reduction of the amount of catalyst that is necessary for the isomerization stage and for the transalkylation stage. Actually, the process according to this invention makes it possible to limit the losses during the isomerization stage, with the use of a catalyst that comprises an EUO-structural-type zeolite. In addition, the stability of the isomerization catalyst is improved relative to the catalysts of the prior art. Finally, the process according to the invention, which combines 3 stages, makes it possible to treat the feedstocks that comprise aromatic compounds with 8 carbon atoms with a large percentage of ethylbenzene, whereby the stages of isomerization and transalkylation make it possible to convert at least a portion of the ethylbenzene that is contained in the feedstock into benzene and xylenes, which makes it possible to increase the final benzene yield. The benzene that is produced with the process according to the invention then has high purity and can advantageously be used in a zone for alkylation of the benzene by mono-olefins for the synthesis of phenylalkanes (linear alkyl benzene, LAB).
The invention relates to a process for the production of at least one isomer of xylenes starting from a feedstock that comprises aromatic hydrocarbons with 8 carbon atoms per molecule, such as orthoxylene, metaxylene, paraxylene, and ethylbenzene, with a desired isomer yield that is improved relative to the process of the prior art with the use in the isomerization zone of a catalyst that comprises at least one EUO-structural-type zeolite.
The feedstock that is to be treated comprises xylenes and ethylbenzene. The process according to the invention advantageously makes it possible to convert feedstocks that comprise a large amount of ethylbenzene. Thus, the feedstocks that are to be treated are such that the ethylbenzene content relative to the overall content of aromatic compounds with 8 carbon atoms per molecule is greater than 25% by weight and even more preferably greater than 35% by weight. The feedstock can comprise any other compound, such as aromatic compounds with 9 carbon atoms, of toluene. It is possible to add these compounds to the feedstock in the form of fresh products or in the form of recycled products. The feedstock can be introduced into the separation zone of the xylenes when the feedstock comprises in large part xylenes and ethylbenzene. In the case where the feedstock contains C7- and C9-aromatic compounds, the feedstock can then be introduced, for example, directly into the transalkylation zone, where the conversion of the C7- and C9-aromatic compounds into xylenes takes place, whereby said xylenes are then sent into a zone for separating xylenes. Furthermore, the feedstock can be introduced into one of the distillation zones that treat the effluent resulting from transalkylation, based on the components of said feed stock.
The process according to the invention thus comprises the following stages:
a) The feedstock is introduced into at least one zone for separating xylenes, and a first fraction that is high in desired isomer and a second fraction that is low in desired isomer are recovered, said second fraction comprising the majority of the other isomers as well as the majority of the ethylbenzene,
b) at least a portion of said fraction that is low in desired isomer and that is drawn off from the separation zone into at least one catalytic zone for isomerization of aromatic compounds with 8 carbon atoms is treated to convert at least a portion of the ethylbenzene into xylenes and to recover an effluent that comprises paraxylene, orthoxylene, metaxylene and ethylbenzene,
c) a feedstock that comprises a portion of said effluent of the isomerization zone, aromatic hydrocarbons with 9 carbon atoms per molecule, of toluene, is treated in at least one catalytic transalkylation zone to recover an effluent that comprises benzene, toluene, C9-aromatic hydrocarbons, orthoxylene, paraxylene and metaxylene,
d) the effluent of the transalkylation zone is treated in at least one distillation zone from which are drawn off a benzene-rich fraction, a toluene-rich fraction, a xylene-rich fraction and a fraction that contains C9-aromatic hydrocarbons,
e) at least a portion of the xylene-rich fraction that is drawn off from the distillation zone is recycled in the zone for separating xylenes that is described in a).
The process according to the invention is characterized in that the catalyst that is used in the isomerization zone that is described in b) comprises at least one EUO-structural-type zeolite and at least one element of group VIII of the periodic table.
The catalyst that is used in the isomerization stage thus comprises at least one EUO zeolite, i.e., the EU-1, TPZ-3 and ZSM-50 zeolites.
The EUO-structural-type EU-1 zeolite, already described in the prior art, has a monodimensional microporous network, whose pore diameter is 4.1xc3x975.7 xc3x85 (1 xc3x85=1 angstrom=1.10xe2x88x9210 m) (xe2x80x9cAtlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996). On the other hand, N. A. Briscoe et al. taught in an article in the Zeolites journal (1988, 8, 74) that these monodimensional channels have side pockets with a depth of 8.1 xc3x85 and a diameter of 6.8xc3x975.8 xc3x85. The synthesis mode of the EU-1 zeolite and its physico-chemical characteristics were described in the EP-B 1-42 226 patent.
U.S. Pat. No. 4,640,829 relates to the ZSM-50 zeolite, which, according to xe2x80x9cThe Atlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996, has the EUO-structural type.
Patent EP-A1-51 318 relates to the TPZ-3 zeolite, which has the EUO-structural type according to xe2x80x9cThe Atlas of Zeolites Structure Types,xe2x80x9d W. M. Meier and D. H. Olson, 4th Edition, 1996.
The first stage of the process according to the invention comprises the treatment of the feedstock in a zone for separating xylenes.
The separation zone comprises at least one adsorption zone in which the majority of the desired isomer or the majority of the undesirable isomers are adsorbed. In an implementation of the invention, the separation zone makes it possible to recover the paraxylene, i.e., the paraxylene is adsorbed and recovered as an extract. Other implementations make it possible to recover the metaxylene and the orthoxylene as an extract based on the selected adsorbent. The adsorption zone operates in a simulated moving bed and comprises at least one zeolitic adsorbent bed that operates in the presence of a suitable desorption solvent and under adsorption conditions such that in the case where an effort is made to recover paraxylene, a first fraction that contains solvent, metaxylene, ethylbenzene and orthoxylene and a second fraction that contains solvent and basically paraxylene are obtained. The paraxylene that is obtained generally has a purity of between 75 and 99.9%. The first fraction is distilled to separate the solvent, on the one hand, and the metaxylene, orthoxylene and ethylbenzene mixture, on the other hand, and the second fraction is also distilled to recover the paraxylene, on the one hand, and the solvent, on the other hand. The first fraction is then sent into the zone for isomerization of aromatic alkyls. The solvent can be recycled at the inlet of the separation zone.
The simulated moving bed can be a simulated countercurrent bed or a simulated co-current bed.
The solvent of elution or desorption is selected from among toluene and paradiethylbenzene. Other solvents such as methyl tert-butyl ether (MTBE) or diisopropyl ether (DIPE) can also be used.
The specific adsorbents of the paraxylene comprise at least one zeolite that is selected from among the X, Y zeolites, whose exchangeable sites are occupied by at least one alkaline cation or alkaline-earth cation, such as, for example, barium and potassium.
The adsorbents that make it possible to absorb selectively the metaxylene and the orthoxylene are, for example, the Y zeolites that are exchanged with at least one element that is selected from among Li, Na, Be, Mg, Ca, Sr, Mn, Cd, Cu, Ni to obtain a raffinate that comprises the paraxylene.
In a particular implementation, the xylene separation zone comprises a crystallization zone downstream from the adsorption zone. Thus, for example, the paraxylene that is recovered after distillation of said second fraction is sent into a crystallization zone at a temperature of between +10 and xe2x88x9225xc2x0 C., and, on the one hand, a mother liquor that is recycled to the adsorption zone is obtained, and, on the other hand, paraxylene crystals that are saturated with mother liquor are obtained, then paraxylene crystals with a very high degree of purity, or generally purity greater than 99.3% and preferably greater than 99.7%, are washed with a suitable washing solvent.
As a solvent for washing crystals, it is possible to use, for example, n-pentane, water, purified paraxylene or toluene. Preferably, either the same solvent for the desorption and for the washing of crystals, such as for example, toluene, is used, or a fraction of purified paraxylene is used.
The first distilled fraction that is recovered after the stage for separating xylenes that comprises orthoxylene, metaxylene and ethylbenzene is treated in an isomerization zone. The isomerization stage is used in the presence of a catalyst that comprises an EUO-structural-type zeolite, for example the EU-1 zeolite. The EUO-structural-type zeolite is at least in part in acid form and comprises silicon and at least one T element that is selected from the group that is formed by aluminum, iron, gallium and boron, preferably aluminum, with an overall Si/T atomic ratio that is greater than 5.
The catalyst also comprises at least one matrix that comprises at least one compound that is selected from the group that is formed by clays, magnesia, aluminas, silicas, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates and silica-aluminas. Preferably, the matrix is alumina. The catalyst comprises at least one element of group VIII of the periodic table, preferably selected from among platinum and palladium, and can also optionally contain at least one metal that is selected from among the metals of groups IIIA and IVA, preferably selected from among tin and indium and optionally sulfur.
The isomerization catalyst comprises by weight relative to the total catalyst mass:
from 1 to 90% by weight of at least one EUO-structural-type zeolite, preferably 3 to 60% and even more preferably 4 to 40%,
from 0.01 to 2% by weight of at least one metal of group VIII, preferably 0.05 to 1%,
optionally from 0.01 to 2% of at least one additional element that is selected from groups IIIA and IVA of the periodic table, preferably 0.05 to 1%,
optionally sulfur,
a binder that ensures the addition by weight to 100% of the catalyst.
According to a preferred embodiment of the invention, a catalyst will be used that comprises an EUO-structural-type zeolite whose crystal size is less than 5 micrometers (xcexcm), preferably less than 0.5 xcexcm, and even more preferably less than 0.2 xcexcm. These crystals or crystallites are often grouped in aggregates that have a grain size such that the value of Dv.90 is less than or equal to 500 xcexcm, often less than 400 xcexcm and most often less than 200 xcexcM and preferably less than 50 xcexcm. The size of the aggregates is determined by grain size with laser diffraction. This measurement is carried out on the zeolite powder that is suspended in water. After a first measurement, the suspension is subjected to ultrasonic waves for thirty seconds, and then a new measurement is carried out. The ultrasonic waves that are used are characterized by a power of 50 W and a frequency of 50 kHz. This procedure is repeated until the result no longer varies (at xc2x15%). The size distribution of the aggregates that is defined by volume is calculated starting from light signals that are collected by the detectors and with the Fraunhofer theory. Dv,X is defined as being the equivalent sphere diameter such that X% by volume of the aggregates has a size that is smaller than said diameter. These characteristics will be obtained directly during the synthesis of the zeolite and/or by any method that makes it possible to reduce the size of the aggregates such as, for example, the post-synthesis grinding or else a suitable mixing before shaping.
The catalyst is preferably prepared as described in the patent application of the applicant FR 97/16,456, incorporated here with reference. Thus, the catalyst preferably has a metal dispersion of group VIII that is between 50 and 100% and more preferably between 60 and 100% and even more preferably between 70 and 100%, whereby a macroscopic distribution coefficient of said metal of group VIII is between 0.7 and 1.3, and preferably between 0.8 and 1.2. Preferably, the catalyst is shaped in the form of balls or extrudates and has a mechanical resistance such that the bed crushing value is greater than 0.7 MPa.
The isomerization zone is operated at a temperature of about 300xc2x0 C. to 500xc2x0 C., preferably about 320xc2x0 C. to 450xc2x0 C. and even more preferably about 350xc2x0 C. to 420xc2x0 C., at a partial hydrogen pressur 0.3 to 1.5 MPa, preferably about 0.4 to 1.2 MPa, at a total pressure of about 0.45 to 1.9 NPa, preferably about 0.6 to 1.5 MPa, at a PPH (feedstock weight/catalyst weight/hour) of about 0.25 hxe2x88x921 to 30 hxe2x88x921, preferably about 1 to 10 hxe2x88x921, and very often 2 hxe2x88x921 to 7 hxe2x88x921. The hydrogen that is introduced for carrying out isomerization can be recycled in said isomerization zone.
In a particular implementation of the isomerization stage, the isomerization zone comprises a recycling as described in the patent application of the applicant FR 98/04 526, incorporated here with reference. The process then comprises at least one distillation zone between the isomerization zone and the transalkylation zone, to recover, after the fraction that comprises the light compounds (with a boiling point less than 80xc2x0 C.) is eliminated, a fraction that contains in large part the aromatic compounds that contain at least 8 carbon atoms per molecule and that is sent into the transalkylation zone to recover a fraction that comprises at least one compound with a boiling point of about 80xc2x0 C. to 135xc2x0 C. and more particularly at least one of the compounds that is selected in the group that consists of naphthenes with 8 carbon atoms per molecule, the paraffins with 8 carbon atoms per molecule, benzene and toluene, at least one of the compounds of said fraction, isolated from the entire fraction by treatment in at least one distillation zone that can be recycled at the inlet of the isomerization zone.
The mass percentage of recycled compounds relative to the total feedstock that enters the isomerization zone is between 0.01 and 20%. Actually, surprisingly enough, the fact of recycling at least one compound with a boiling point of between 80xc2x0 C. and 135xc2x0 C. makes it possible to reduce the parasitic reactions of the isomerization and, moreover, makes it possible to reduce the volume of the feedstock of the transalkylation zone, which, combined with the performance levels of the catalyst according to the invention, ensures considerable savings for the device. The naphthenes are preferably recycled in the isomerization zone, whereby the other compounds of the fraction with a boiling point of between 80xc2x0 C. and 135xc2x0 C. such as toluene and the paraffins can be recovered at this level of the process.
The effluent resulting from the isomerization zone that comprises the three isomers of xylenes in a ratio that is basically close to that of the thermodynamic equilibrium and that comprises ethylbenzene is then sent into a catalytic transalkylation zone. Aromatic compounds with 9 carbon atoms are added to the feedstock of the transalkylation zone of the toluene either by recycling or by adding fresh products. The transalkylation zone makes it possible both to transform the C9-aromatic compounds and the C7-aromatic compounds such as toluene into xylenes and to dealkylate the majority of the ethylbenzene into benzene and therefore to convert the majority of the ethylbenzene that is present in the feedstock into benzene while obtaining a total paraxylene yield that is improved relative to the processes of the prior art.
The transalkylation stage can be used in the presence of any catalyst that is known to one skilled in the art. In particular, it is possible to use a catalyst based on mordenite and at least one metal of group VIIB of the periodic table, such as, for example, rhenium. The catalyst can also contain at least one element that is selected from groups VIII, IVB, III and VIB of the periodic table, preferably at least one element that is selected from groups IVB and VIII, preferably selected from among nickel and tin.
In general, the catalyst comprises 40 to 90% by weight of mordenite zeolite that has an Si/Al ratio that is greater than 35, from 0.01 to 5% by weight of at least one metal of group VIIB, whereby the addition by weight consists of binder, selected from among, for example, clays, magnesia, aluminas, silicas, titanium oxide, boron oxide, zirconia, aluminum phosphates, titanium phosphates, zirconium phosphates and silica-aluminas. Preferably the binder is alumina.
Surprisingly enough, the transalkylation catalyst of this invention makes possible the transalkylation of the aromatic compounds and also the dealkylation of the aromatic compounds, such as, for example, the ethylbenzene, to obtain benzene in a large amount and therefore to convert the majority of the ethylbenzene that is present in the feedstock.
The transalkylation zone is used at a temperature of between 250 and 600xc2x0 C., preferably between 330 and 500xc2x0 C., under a pressure of between 1 and 6 MPa, preferably between 2 and 4.5 MPa, with a PPH that is expressed by kilogram of feedstock that is introduced per kilogram of catalyst and per hour, between 0.1 hxe2x88x921 and 10 hxe2x88x921 and preferably between 0.5 and 4; a hydrogen to hydrocarbon molar ratio of between 2 and 20 and preferably between 3 and 12. The hydrogen that is introduced for the transalkylation can be recycled in the transalkylation zone.
The effluent of the transalkylation zone that comprises 3 isomers of xylenes with a composition that is close to the one at equilibrium, benzene, a small amount of ethylbenzene, toluene and aromatic compounds with 9 carbon atoms per molecule that are not transformed is then freed of its lightest components that comprise up to 6 carbon atoms per molecule, whereby the bottom effluent is then fractionated by treatment in at least one distillation zone to recover a fraction that comprises in large part benzene, a fraction that comprises in large part toluene, whereby said fraction can be at least partly recycled to the transalkylation zone, a fraction that comprises in large part xylenes and a small amount of ethylbenzene that is sent at least partly into the zone for separating xylenes, a fraction that comprises in large part aromatic compounds with 9 carbon atoms that can be at least partly recycled into the transalkylation zone. The process preferably comprises at least one zone for separate distillation by recovered fraction. Thus, the process preferably comprises a distillation zone for recovering light compounds, a distillation zone for recovering benzene, a distillation zone for recovering toluene, a distillation zone for recovering xylenes, a distillation zone for recovering C9-aromatic compounds.
The fraction that comprises in large part the aromatic compounds with 8 carbon atoms is then sent as a feedstock into the zone for separating xylenes to which it is possible to add a feedstock as defined above.
In a particular implementation of the invention, the benzene that is produced will be sent into a reactor that contains a suitable catalyst for the production of phenylalkanes with alkylation by mono-olefins. The aromatic alkylation stage can be carried out in the presence of any catalyst that is known to one skilled in the art. It is possible to use an acid-type catalyst such as HF or a Friedel-Craft-type catalyst, in particular with an AlCl3 base. It is also possible to use solid catalysts based on cancrinite, gmelinite, mordenite, offretite, ZSM-12 or EUO catalysts.
In the case of a zeolitic catalyst, a catalyst with an EUO zeolite base, such as EU-1, shaped with a binder such as alumina, is preferably used.
During the alkylation stage, the benzene that is obtained from the effluent of the transalkylation is preferably reacted in a reaction zone with a feedstock that contains at least one linear olefin that comprises 9 to 16 carbon atoms per molecule, preferably 10 to 14 carbon atoms per molecule, in contact with a catalyst based on an EUO-structural-type zeolite that has the characteristics defined above, then the product that is obtained is fractionated to collect separately a first fraction that contains unconverted benzene, a second fraction that contains at least one unconverted C9-C16 linear olefin (preferably C10-C14), a third fraction that contains 2-, 3-, 4-, 5- and 6-phenylalkanes and a fourth fraction that contains at least one poly-alkylbenzene (or poly-alkylbenzene fraction), whereby the latter is then at least partly recycled to said zone for alkylating benzene where it therefore reacts with benzene in contact with said catalyst to be transalkylated at least in part, and a mixture of 2-, 3-, 4-, 5- and 6-phenylalkanes is collected.
Thus, the reactions for alkylation of benzene and for migration of the alkyl groups to the benzene core take place together in the same reaction zone (i. e, in the same reactor) in the presence of the same catalyst. The first fraction that contains unconverted benzene is preferably at least partly recycled to said reaction zone. Likewise, preferably the second fraction that contains at least one unconverted C9-C16 linear olefin (usually C10-C14) is at least partly recycled to said reaction zone.
The recycled portion of the fourth fraction preferably contains basically at least one dialkylbenzene and is preferably essentially free of heavy alkyl-aromatic compounds that optionally can be eliminated by fractionation.
This variant of the invention can be carried out at a temperature that is less than 300xc2x0 C., for example less than 60xc2x0 C., under a pressure of 1 to 10 MPa, with a liquid hydrocarbon flow rate (volumetric flow rate) of about 0.5 to 50 volumes per volume of catalyst and per hour and with a benzene/olefin molar ratio of between 1 and 20.